1. Technical Field
This disclosure relates to an image forming apparatus including a recording head that ejects a liquid droplet; and a method for correcting a displacement of the impact position of a liquid droplet to be ejected from the recording head.
2. Description of the Related Art
Image forming apparatuses (e.g. printers, fax machines, copiers, and multifunction machines having functions of these machines) are known that perform image formation by ejecting a liquid (a recording liquid) such as ink onto a medium with use of, e.g., a liquid ejection device while transporting the sheet. The liquid ejection device comprises a recording head including a liquid ejection head (liquid droplet ejection head) for ejecting a droplet of the recording liquid (ink). It is to be noted that the term “medium” as used herein is hereinafter also referred to as a “sheet”, which may be paper or may be made of other materials. The terms “to-be-recorded medium”, “recording medium”, “transfer material”, and “recording sheet”, may be used as synonyms. The terms “recording”, “printing”, and “imaging” may be used as synonyms for the term “image formation”.
The term “image forming apparatus” as used herein indicates an apparatus that forms images by ejecting liquid onto media such as paper, strings, fibers, cloth, leather, metal, plastic, glass, wood, and ceramics. The term “image formation” as used herein indicates not only forming images that have meanings, such as characters and figures, on a medium, but also forming images that do not have meanings, such as patterns, on a medium. The image forming apparatus may include a textile printing apparatus and an apparatus for printing interconnects. The term “liquid” as used herein is not limited to recording liquid, but includes any liquid that can be used for image formation.
A problem with such a liquid ejection type image forming apparatus, especially one that reciprocates a carriage with a recording head for ejecting liquid droplets to print images in opposite printing directions, i.e., in the forward path and the backward path, when the apparatus draws images such as a ruled line, a ruled line drawn by the head moving in one direction is often misaligned with a line drawn by the head moving in the opposite direction.
Usually, in the case of inkjet recording apparatuses, a user manually outputs a test chart for correcting the misalignment of ruled lines and selects and enters an optimum value, thereby correcting the ejection timing based on the entered value. However, a wrong interpretation of the test result and an input error by a user unfamiliar with the apparatus might result in a greater misalignment.
With regard to liquid ejection type image forming apparatuses, Japanese Patent Laid-Open Publication No. 4-39041 (Patent Document 1) discloses an image forming apparatus that corrects density irregularities. This apparatus prints a test pattern on a recording medium or a transport belt, reads the color data of the test pattern, and changes the head driving conditions based on the read data, thereby correcting density irregularities.
Japanese Patent Registration NO. 3838251 (Patent Document 2) discloses an inkjet recording apparatus that detects a nozzle of a liquid ejection head having an ejection failure. The inkjet recording apparatus forms a test pattern of mixed color dots on a support/transport member using a cyan ink, a magenta ink and a yellow ink, reads the mixed color dots by using an RGB sensor, and detects a failing nozzle based on the read result.
Japanese Patent Laid-Open Publication No. 2005-342899 (Patent Document 3) discloses an inkjet recording apparatus that records either one or a combination of a failed nozzle detection pattern for detecting a failed nozzle and a color misregistration detection pattern for detecting an ink color misregistration on a part of a transport belt, detects the test pattern using an imaging unit such as a CCD, and performs correction based on the read result.
Meanwhile, with regard to electrophotographic image forming apparatuses using toner, Japanese Patent Laid-Open Publication No. 5-249787 (Patent Document 4) discloses an image forming apparatus that detects the density of a toner image using a light emitting device and a light receiving device. The light emitting device includes a light receiving element for receiving specular reflection light and a light receiving element for receiving scattered reflection light. The image forming apparatus forms toner images of different characteristics on a photoreceptive drum and detects the respective toner images.
Japanese Patent Laid-Open Publication No. 2006-178396 (Patent Document 5) discloses an image forming apparatus that detects the amount of attached toner based on a detection result of a sensor which is capable of detecting specular reflection light and diffused reflection light at the same time.
In the case of forming a test pattern on a transport belt and detecting the color of the test pattern or reading the test pattern as in Patent Documents 1-3, some combination of the color of the transport belt and the color of the ink has a small difference in color, which makes it difficult to accurately read the test pattern. In this case, for accurate color detection, it is necessary to use an expensive detecting unit including, e.g., a light source that emits lights of different colors and wavelengths. For example, in the case where an electrostatic transport belt is used that includes an insulation layer forming the front surface and an intermediate resistance layer forming the back surface and containing carbon for providing conductivity, because the external color of the electrostatic belt is black, it is difficult to detect black ink when detecting a test pattern based only on reflectance or the image read by an imaging unit, thereby failing to provide accurate detection.
More specifically, with regard to the image forming apparatus of Patent Document 1 that corrects density irregularities, because a sensor for detecting colors is used, the detection accuracy is reduced when detecting an ejected ink droplet having a color close to the color of the support/transport member. The sensor needs to have filters one for each color. An increased number of filters and sensors result in a higher cost. With regard to the inkjet recording apparatus of Patent Document 2 that detects a nozzle failure, because an RGB sensor is used, the detection accuracy is reduced when detecting an ejected ink droplet having a color close to the color of the support/transport member. Increasing the detection accuracy limits the number of combinations of the inks to be used and the transport member. If a laser beam is used which scans a very small point, small bits of foreign matter and scratches of the transport member easily affect the detection result, resulting in reduced detection accuracy. The RGB sensor needs to have sensors one for each color, which means an increased cost. With regard to the inkjet recording apparatus of Patent Document 3 using an imaging unit, as in the case of the inkjet recording apparatus of Patent Document 2, the detection accuracy is reduced when detecting an ejected ink droplet having a color close to the color of the support/transport member. Moreover, because an image is processed as a two-dimensional image, a processing system is required which has higher performance than a processing system that processes one-dimensional images. This results in an increase of the cost.
To avoid these problems, a system of detecting the amount of attached toner as used in the electrophotographic image forming apparatuses of Patent Documents 4 and 5 may be used. However, because toner particles maintain their shapes even when in contact with one another, it is possible to detect toner particles densely accumulated in the shape of a rectangular line. If this system is used in a liquid ejection type image forming apparatus, detected droplets only have a small level difference from noise, which prevents highly accurate detection of a test pattern.
In the case where an optical sensor reads a test pattern formed on plain paper as a recording medium into which ink can penetrate, the test pattern blurs due to penetration of ink, which prevents accurate detection of the impact positions of ink droplets.